The substantia gelatinosa (SG) of spinal cord is located in such a way that it may modulate transmission of sensory impulses from peripheral nerves onto the large cells of dorsal horn. We have now developed methods to stimulate some SG axons in Lissauers tract (LT), to record from some SG cells and to investigate the effects of LT stimulation on dorsal roots, dorsal horn cells and ventral root reflexes. Encouraged by the early results which support the idea of SG modulation, we propose now to investigte the role of SG in the modulating effects o stimulation of peripheral nerves, of LT, and of descending pathways. There are some preliminary indications of long lasting effects of SG activity. We have discovered a form of plasticity in which deafferentation changes the receptive fields of large dorsal horn cells. We propose to study the long-term effects of stimulation and deafferentation on SG activity and dorsal horn activity. The overall objective is to discover short-and long-term mechanisms of modulation of sensory transmission.